Directional borehole operations, such as directional drilling, involve varying or controlling the direction of a downhole tool (e.g., a drill bit) in a wellbore to direct the tool towards a desired target destination. In directional drilling, for example, the direction of a drill bit is controlled to direct the bit, and the resultant wellbore, towards a desired target destination.
Various techniques have been used for adjusting the direction of a tool string in a borehole. Slide drilling employs a downhole motor and a bent housing to deflect the borehole. In slide drilling, the direction of the borehole is changed by using the downhole motor to rotate the bit while drill string rotation is halted and the bent housing is oriented to deflect the bit in the desired direction. Slide drilling systems are subject to various problems related to halting drill string rotation. For example, a non-rotating drill string is subject to sticking in the wellbore.
In contrast to slide drilling systems, rotary steerable systems allow the entire drill string to rotate while changing the direction of the borehole. By maintaining drill string rotation, rotary steerable systems overcome various deficiencies of slide drilling. A tool for controlling deflection in a rotary steerable system (i.e. a rotary steerable tool) generally includes a shaft that rotates with the drill string, and a housing surrounding the shaft that includes a device that applies a deflection force to the shaft. By deflecting the shaft, the direction of the downhole end of the shaft is changed.